The thermal power system tends to raise the steam temperature of a steam turbine in order to make the generating efficiency much higher. As a result, high temperature characteristics demanded for the forging heat resistant steel to be used for the steam turbine also become much stricter.
There have been proposed many heat resistant steels for forging to be used for steam turbines.
It is necessary to improve a long creep rupture life of a forging heat resistant steel which is used for the steam turbine in order to contribute to further improvement of the generating efficiency. Materials configuring rotating parts and large-size forged parts, such as the turbine rotor of the steam turbine, are required to have excellent creep ductility and toughness from a viewpoint of prevention of breakage at the time of operation.
When the forging heat resistant steel receives prolonged aging and prolonged creep deformation at a high temperature, creep rupture ductility and toughness might be reduced. If degradation of such characteristics occurs in the turbine rotor which is a large rotating structural component, an operational risk increases. A conventional forging heat resistant steel is mainly under study for a compositional improvement from a viewpoint of improving the creep rupture life, but a compositional improvement has not been studied sufficiently considering particularly creep ductility and toughness.
It is very difficult to achieve both improvement of a prolonged creep rupture life and improvement of creep rupture ductility and toughness.